1. Technical Field
The present invention relates to the decoding of radio transmission channels carrying coded digital information. More precisely, the invention relates to the acquisition of the carrier frequency of a signal modulated with a quadrature single-carrier continuous modulation, for example, a quadrature amplitude modulation known as QAM.
2. Description of Related Art
Solutions currently used to carry out the acquisition and monitoring of the carrier frequency are based on phase-locked loops.
More precisely, a first category of solution uses a phase-locked loop having a wide frequency locking range for the acquisition of the carrier frequency, then a narrower frequency locking range when monitoring the carrier frequency. However, such a solution has a disadvantage in that the acquisition may fail with an input signal of poor quality.
A second category of solution uses a phase-locked loop having a narrow frequency locking range in order to be able to carry out the acquisition of a poor quality signal and in parallel uses a frequency scanning algorithm in order to acquire a signal exhibiting a significant frequency shift from the carrier frequency. However, this second solution has a disadvantage of requiring considerable locking time.
A need accordingly exists in the art to address the foregoing disadvantages.
More specifically, a need exists to be able to estimate and correct significant frequency shifts, for example of the order of hundreds of kHz even with a poor quality input signal, for example exhibiting a signal-to-noise ratio of the order of 20 dB for a 64 QAM modulation, and doing so for a short time, for example of the order of a hundred milliseconds.